opencl: general flash attention decode performance optimizations (#25366)

• opencl: vec flash-attention decode kernels for f16/q8_0/q4_0 KV

• opencl: improve non FA KQ mv kernels

• opencl: tweaks for multiquery FA

• opencl: some tweaks for FA q1 kernels

• opencl: FA with DK=DV=512 for gemma-4

• opencl: various fixes

• opencl: cleanup

• opencl: fix FA decode crash for DK=512 (gemma-4)

The DK=512 decode-only program does not create the f32_f16 prefill kernel, so the compiled check in ensure_fa_variant never hit and supports_op gave inconsistent answers for the same op. block_n is also unset for DK=512 decode; guard it to avoid an out-of-range read at dispatch.

• opencl: run DK=512 FA decode on CPU

DK=512 decode is bandwidth-bound and faster on the CPU than the GPU, increasingly so with depth. Decline it in supports_op; prefill stays on the GPU.

• opencl: compile MQ_GQA=8 FA kernels in a minimal program

The full program compiled with -D MQ_GQA=8 runs the Adreno compiler out of memory at DK>=256. Only the vec_mq kernels are used from this program, so compile it with FA_MQ_ONLY, which excludes everything else. Also include the program name in the compile error log.

• opencl: remove stray token in flash_attn_f32_f16.cl

A stray "." broke the f32_f16 program build.

• opencl: split f16-KV FA decode finer (FD_KV_PER_SPLIT_F16)

The 2048 default under-fills the GPU on single-query f16-KV decode; use 512 for f16 KV to get more splits. Quantized KV keeps 2048.

Co-authored-by: Li He [email protected]